In many modern hearing aids, the space available within the hearing aid device is usually quite limited and designers of such products are loath to add more components to provide wireless power or charging to the hearing aid. This is particularly true for hearing aids placed within the ear canal, where space is a premium.
Normally, a hearing aid picks up sound with its microphone then amplifies the sound for the wearer to hear that sound more clearly. In many modern hearing aids, a telecoil is used as the input source instead of (or in addition to) the microphone, such as to improve hearing aid function when a person is using a telephone with a dynamic speaker. In this manner, the hearing aid can pick up a magnetic signal which represents sound produce by the speaker in the phone. In some cases, a telecoil is employed to bypass the microphone in the hearing aid to avoid a feedback between the microphone and the telephone speaker when the hearing aid wearer is speaking on a telephone. This is accomplished by avoiding the sound from the speaker in the phone from being picked up by the microphone by muting the microphone circuit and instead using the telecoil to pick up the magnetic field variations from the telephone speaker and using that signal for the audio input of the hearing aid system.